The present invention relates to the improvement of a microwave plasma source, and more particularly to power saving features in such microwave plasma sources.
Due to an increasing degree of integration in semiconductor integrated circuits, manufacturing apparatus such as etching apparatus, ion implantation apparatus and deposition apparatus are required to have further enhanced performance. In order to meet such a requirement, a microwave plasma etching apparatus has been proposed in, for example, Japanese Patent Publication No. 53-34463, which is owned by the same assignee as the present application. FIG. 1 shows the basic structure of the microwave plasma etching apparatus as proposed. The operation of the apparatus will be briefed in the following. The microwave energy at a frequency of 2.45 GHz generated by a microwave generator such as a magnetron is propagated through a round waveguide 2 formed of copper, for example, to a vacuum room 5 which consists of a discharge tube 3 and a vacuum chamber 4 made of alumina or quartz. The vacuum room 5 is maintained in a vacuum state by an evacuation system 6. The vacuum chamber 4 is provided with a substrate stage 7, on which a sample 8 such as a semiconductor wafer to be etched is placed. Around the waveguide 2 enclosing the discharge tube 3, there is disposed an electromagnet (solenoid) 9, and behind the substrate stage 7, there is disposed a permanent magnet 10. In such an arrangement, a spatial region enclosed by the vacuum chamber 5 and the discharge tube 3 within the vacuum room 5 forms a discharging space 11, in which an electric field caused by the microwave and a magnetic field caused by the electromagnet 9 and the permanent magnet 10 are provided. In this state, when a discharge gas such as CF.sub.4 in a partial pressure of around 1.times.10.sup.-3 Torr is conducted into the discharging space 11, it is reduced to the plasma state by the multiplier effect on the electric and magnetic fields. An ion sheath is formed between the generated plasma and the sample 8, and the sample 8 is hit by ions in the plasma. Consequently, the surface of the sample 8 is etched by ions.
A microwave plasma ion implantation apparatus has been proposed in Japanese Patent Application No. 54-68952 filed by the same assignee of the present application. FIG. 2 shows the basic structure of the microwave plasma ion implantation apparatus. The operation of the apparatus will be briefed hereunder. The microwave energy at a frequency of 2.45 GHz propagates through a rectangular wave guide 2', a choke flange 12, a ridged wave guide 13 formed of copper, and a vacuum sealing insulator plate 14 made of alumina to a discharging space 11 within a discharge tube 3. A microwave electric field is produced in the discharging space between ridged electrodes 15. Around the ridged electrodes 15, there is provided an electromagnet 9 which produces the magnetic field in the discharging space 11. A vacuum room 5 consisting of the discharge tube 3 and a vacuum chamber 4 is maintained in a vacuum state by an evacuation system 6. When a discharge gas such as PH.sub.3 in a partial pressure of around 8.times.10.sup.-6 Torr is introduced into the vacuum chamber 5, it is reduced to plasma state by the multiplier effect on the electric and magnetic fields in the discharging space 11. Ions in the produced plasma are extracted by extraction electrodes 16 disposed within the vacuum chamber 4. Extracted ions are introduced through a mass separator 17 to a process chamber 18. Then, the ions are projected onto the sample 8 such as a semiconductor wafer through a slit system 19. Consequently, an implanted layer is formed on the surface of the sample 8.
The above mentioned etching apparatus and the ion implantation apparatus commonly have a plasma generating means. Both types of apparatus reduce discharge gas in the discharging space 11 into plasma state, then extract ions extracted from plasma are used to make fine patterns on the surface of the sample substrate 8. Both types of apparatus utilize the multiplier effect on the microwave electric and magnetic fields for reducing the discharge gas to the plasma state. The magnetic field needs to be produced in the axial direction of the wave guides 2 and 13 in an intensity of around 1000 gauss. The magnetic field is produced by the electromagnet 9 as mentioned above, and it generally requires a few kW of power (e.g. 60 V, 20 A) to obtain the magnetic field at this level. The foregoing etching apparatus has the discharging space 11 of a large diameter D, and it is necessary to produce a high density plasma uniformly in the radial direction. Therefore, the majority of the power for the etching apparatus is likely to be consumed by the electromagnet 9. Furthermore, it can generally not be avoided that a number of the samples 9 must be subject to the fine process simultaneously for increasing productivity. In order to meet this requirement, it is necessary to develop a microwave plasma etching apparatus or a microwave plasma ion implantation apparatus each having the discharging space 11 with a large diameter D. However, as the diameter D of the discharging space 11 increases, the diameter of the electromagnet 9 also increases proportionally. Since creation of plasma needs the magnetic field at a certain level, it is obvious that a larger diameter D of the discharging space will result in a very large power consumption in the electromagnet 9. The increased power consumption does not meet the contemporary requirement for energy saving, and it has been desired in the industry to resolve this problem.